Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Kesterite-based solar cells are attracting considerable attention in recent years, owing to the reduced toxicity and greater abundance of their constituent elements. In this brief review, we discuss the current status of this important technology by focusing on three key aspects of the device: (i) the interface between the kesterite absorber and the Mo back contact, (ii) the kesterite absorber bulk defects and grain boundaries and (iii) the interface between the kesterite absorber and the buffer layer. By identifying key issues to be addressed, we provide suggestions for their potential improvement and future research. Copyright © 2016 John Wiley & Sons, Ltd.

The current status and future prospects of kesterite solar cells: a brief review

Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(Sx,Se1–x)4 compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In1–y,Gay)Se2 and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open-circuit voltage (Voc) never better than 60% of the Voc max, which is expected from the Shockley-Queisser radiative limit (S-Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the Voc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain-to-grain and other non-uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the Voc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S-Q limit. The implications and future ways for improvement are discussed.

Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells?

Application of zinc-blende-related chalcogenide absorbers such as CdTe and Cu(In,Ga)Se2 (CIGSe) has enabled remarkable advancement in laboratory- and commercial-scale thin-film photovoltaic performance; however concerns remain regarding the toxicity (CdTe) and scarcity (CIGSe/CdTe) of the constituent elements. Recently, kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) materials have emerged as attractive non-toxic and earth-abundant absorber candidates. Despite the similarities between CZTSSe and CIGSe/CdTe, the record power conversion efficiency of CZTSSe solar cells (12.6%) remains significantly lower than that of CIGSe (22.6%) and CdTe (22.1%) devices, with the performance gap primarily being attributed to cationic disordering and associated band tailing. To capture the promise of kesterite-like materials as prospective “drop-in” earth-abundant replacements for closely-related CIGSe, current research has focused on several key directions to control disorder, including: (i) examination of the interaction between processing conditions and atomic site disorder, (ii) isoelectronic cation substitution to introduce ionic size mismatch, and (iii) structural diversification beyond the zinc-blende-type coordination environment. In this review, recent efforts targeting accurate identification and engineering of anti-site disorder in kesterite-based CZTSSe are considered. Lessons learned from CZTSSe are applied to other complex chalcogenide semiconductors, in an effort to develop promising pathways to avoid anti-site disordering and associated band tailing in future high-performance earth-abundant photovoltaic technologies.

Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials

Kesterite Cu2 ZnSnS4 is a promising photovoltaic material containing low-cost, earth-abundant, and stable semiconductor elements. However, the highest power conversion efficiency of thin-film solar cells based on Cu2 ZnSnS4 is only about 11% due to low open-circuit voltage and fill factor mainly caused by antisite defects and unfavorable heterojunction interface. In this work, a postannealing procedure is proposed to complete a Cd-alloyed Cu2 ZnSnS4 device. The postannealing to complete the device significantly enhances the performance of the indium tin oxide and promotes the moderate interdiffusion of elements between the layers in the device. As a result of the diffusion of Cu, Zn, In, and Sn, the interfacial electron and hole densities are improved, leading to the achievement of a suitable band alignment for carrier transport. The postannealing also reduces the interface traps and deep-level defects, contributing to decreased nonradiative recombination. Therefore, the open-circuit voltage and fill factor are both improved, and an efficiency over 12% for pure sulfide-based kesterite thin-film solar cells is obtained.

Device Postannealing Enabling over 12% Efficient Solution‐Processed Cu 2 ZnSnS 4 Solar Cells with Cd 2+ Substitution

We report on order–disorder related band gap changes in Cu2ZnSn(S,Se)4 solar cells which are induced by post-annealing. The band gap changes of the absorber are detected utilizing electroreflectance and analyzed by comparison with predictions of the stochastic Vineyard model. This yields a critical temperature of TC=195 °C above which the Cu2ZnSn(S,Se)4 absorber layer is entirely disordered within the Cu–Zn layers of the kesterite unit cell. The temporal evolution of the band gap during annealing shows that the equilibrium value is reached on a timescale in the order of hours, depending on the annealing temperature. In contrast to other experimental techniques, electroreflectance precisely measures the band gap and is not influenced by defect-mediated radiative recombination.

Reversible order-disorder related band gap changes in Cu2ZnSn(S,Se)4 via post-annealing of solar cells measured by electroreflectance

We investigate the impact of order-disorder related band gap changes on finished Cu2ZnSn(S,Se)4 solar cells. Solution-processed solar cells are post-annealed in a tube furnace in order to modify the Cu-Zn disorder within the kesterite unit cell. A decrease in this disorder leads to an increased band gap of the absorber. The latter is detected using electroreflectance spectroscopy. In our experiments the change in the open-circuit voltage follows the change in the absorber band gap. However, the open-circuit voltage deficit of the solar cell remains unaltered. Still, power conversion efficiencies could be increased by 1%, mainly due to an increased open-circuit voltage.

Order-disorder related band gap changes in Cu2ZnSn(S,Se)4: Impact on solar cell performance

In order to identify the impact of the degree of Cu–Zn order in kesterite Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails, we perform photoluminescence (PL), photoluminescence excitation, and time-resolved photoluminescence (TRPL) spectroscopy. The PL lineshape and further PL characteristics such as state filling are analysed as a function of Cu–Zn order. Furthermore, TRPL decays and band tails are quantified. No significant modification in the defect states is caused by changes in Cu–Zn order, meaning that the formation of the defect states is not mainly determined by disorder in the Cu–Zn plane. In regard to band tailing, a small tendency to a decrease in the tailing parameter for the states with a high degree of Cu–Zn order compared to states with a low degree of Cu–Zn order is obvious. However, this reduction is small compared to the reduction of the defect density accompanied by the increase in the degree of Cu–Zn order. Hence, band tails are not mainly formed due to disorder in the Cu–Zn planes.

Impact of the degree of Cu–Zn order in Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails

Luminescence properties of Cu 2 ZnSn (S ,Se ) 4 solar cell absorbers: State filling versus screening of electrostatic potential fluctuations

We investigate the electronic structure and the radiative recombination in wet-chemically fabricated Cu2ZnSn(S,Se)4 solar cell absorbers utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy, focusing especially on the effects of varying Cu content. This includes the impact of the latter on the band gap energy and the change in band gap energy related to the order-disorder transition. Characteristic PL and PLE parameters like the energetic position of the PL maximum and the PL yield as a function of the excitation power as well as the PLE tailing parameter do not depend on composition indicating that the nature of the radiative transition is not altered by the Cu content. However, the band gap energy Eg significantly increases as a function of decreasing Cu content. This increase is more pronounced in the disordered than in the ordered atomic arrangement of Cu and Zn atoms in the Cu–Zn planes of the kesterite crystal structure.

Influence of the Cu Content in Cu2ZnSn(S,Se)4 solar cell absorbers on order-disorder related band gap changes

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